One of the most costly aspects of IC technology is bonding the IC chip to the rest of the circuit package. A solution to this problem comprises automated bonding of metal beam leads to the IC chip, thus avoiding manual wire bonding. Such a technique uses a roll of a laminate that carries to a bonding station a plurality of frames, each of which has many generally planar microscopic, spider-shaped metal fingers known as beam leads. At the bonding station the inner portions of the beam leads are aligned and then simultaneously connected to the bonding sites on the IC chip. The outer portion of the beam leads is then available for bonding to remaining portions of the circuit package.
Such beam leads can be manufactured by a photoresist process. Guild U.S. Pat. No. 4,247,623 discloses such a process and a blank or laminate for use therein having a structure comprising a flexible strip of electrically conductive metal, a layer of resist adhered to one surface of the metal strip, and a layer of resist adhered to the opposite surface of the metal strip. The process includes the steps of imagewise exposing and developing the resists. One developed resist forms a protective coating on the beam lead portions of the metallic layer. The exposed metallic layer is subsequently etched to form the beam leads. The other developed resist forms a window-bearing support spacer holding the beam leads in the desired orientation. The beam leads are maintained in their proper orientation prior to and during bonding by the spacer formed from such resist.
While the three layer blanks and the process provided by Guild represent a significant advance in the art, the materials suggested by Guild for use in his invention are not satisfactory for some high temperature operations. Furthermore, those materials cause stress at the polymer/metal interface resulting in curl of the product. In other words, the beam leads do not stay in a flat plane.
Klein, U.S. Pat. No. 4,792,517 discloses a laminate for the formation of beam leads for IC chip bonding which include a layer of a resist which is used to form a support spacer exhibiting good dimensional integrity and hardness at processing temperatures of 200.degree. C. and higher. The spacer functions to support the leads prior to, during, and after the bonding of the IC chip. While the laminate described in U.S. Pat. No. 4,792,517 provides, under most conditions, superior beam leads. However, in high humidity environments, it undergoes hydrolytic degeneration. Furthermore, materials provided by Klein can lead to curl of the thin laminated product.
One proposed solution to the curl problem recognized in the art is to use a thinner (i.e. narrower) polymer coating to reduce curl. This results in less coverage of metal parts, and the unnecessary plating of gold to exposed metal areas in subsequent steps. The unnecessary plating of gold results in a significant cost increase.
In previous work by the present inventors, they demonstrated that it was possible to provide improvements over the compositions suggested by Guild and Klein. More specifically, the current inventors were able to show that certain soluble polyimides, and the siloxane modified polyimides suggested by Mueller et al, U.S. Pat. No. 4,803,147, provided significant high temperature and hydrolytic stability. However, these materials did not adequately solve the curl problem referred to above.
Now, with the present invention, Applicants have provided laminates which have good high temperature properties, good hydrolytic stability, and significantly reduced curl. Hence, the laminates of this invention represent a significant advance in the art. Stated another way, the laminates of the present invention have a combination of desirable properties which is not attainable with prior art laminates.
In short, a need exists for laminates with good high temperature properties, hydrolytic stability, and reduced curl. This invention satisfies that need.